WASHINGTON — The U.S. Air Force’s top uniformed space official continues to push for what he says is his top budget priority, a follow-on to a space surveillance satellite launched in 2010, but indications are that the effort is not funded in U.S. President Barack Obama’s forthcoming budget request for 2015.

That means the Air Force potentially faces a lengthy gap in capability once the current Space Based Space Surveillance (SBSS) satellite reaches the end of its lifetime.

“We think this satellite will likely hit end of life sometime around 2017,” Gen. William Shelton, commander of Air Force Space Command, said in a Feb. 7 speech here. “The great news is our industry produces satellites that last longer than we think they will. So we’re hopeful that this one does because the follow-on is in question right now. But we have got to sustain this capability.”

In interviews and speeches, Shelton said military space programs fared “reasonably well” in the budget request, to be unveiled and sent to Capitol Hill in early March.

But he also cautioned that the budget agreement reached by Congress in December, which led to the Consolidated Appropriations Act of 2014, did not solve all the problems created by the automatic spending cuts known as sequestration. Specifically, he said, the agreement provided “some relief” in 2014 and 2015 but that 2016 and beyond “still looks to be very problematic for me.”

Shelton said his immediate concern for the 2015 budget is a follow-on to the SBSS Block 10 satellite. Built by Boeing Space and Intelligence Systems, the satellite is part of an emerging space situational awareness program that also includes ground-based sensors to track satellites and orbital debris. Ball Aerospace & Technologies Corp. was a major subcontractor on the SBSS satellite.

“The one thing missing is an SBSS follow-on,” Shelton said in an interview Jan. 27. “And that’s one we’ve got to keep advocating for. That’s a critical capability; we’ve just got to have a resource on orbit that gives us timely visibility of the geosynchronous belt. And we don’t have anything that can do that on the ground. An all-weather, day, night capability other than SBSS. So sustaining that capability is critical.”

SBSS Block 10, which officially commenced operations in April 2013, is designed primarily to keep tabs on the geostationary orbit arc 36,000 kilometers above the equator, which is home to most communications satellites, both military and commercial. The Air Force has long cited the need for a follow-on companion satellite for broader coverage, but budget pressures have forced the service to defer the mission, which is now viewed as more of a replacement than a complement to the existing satellite.

Shelton said the Air Force is working on delivering a more convincing message on the need for the next satellite.

“In some cases, people haven’t quite understood that this really is the only way you get timely information on geo. And the problem is, our radars that go out to geo, that’s a pencil beam. That’s not broad, synoptic coverage. Our optical ground sites, of course, are only nighttime operations and also subject to cloud cover, so this is the only thing that’s going to give you that coverage and if anything happens in geo it’s gonna catch it.”

In a Jan. 14 press release, Boeing said Air Force data show SBSS cut the danger of satellites being lost by two-thirds. “SBSS provides a major advantage to satellite operators who need to protect these valuable space assets that we depend on every day,” Craig Cooning, Boeing Space & Intelligence Systems vice president and general manager, said in the release.

In 2012, the Air Force said it planned to award a contract for a follow-on SBSS satellite in fiscal year 2015, with a launch in 2020. That launch timeline now appears to have slipped to about 2022, sources said.

In 2012, the Air Force said it “accepts the risk” of a gap in its ability to keep tabs on objects in geosynchronous orbit. That scenario would become a reality if the SBSS Block 10 satellite fails before the follow-on satellite is launched.

Steve Smith, director of military space at Ball Aerospace of Boulder, Colo., said a follow-on program could piggyback on the development work for the Block 10 satellite and therefore could be produced more affordably.

“Industry has certainly provided ideas on how to do geostationary space situational awareness affordably,” Smith said.

Senior Pentagon officials have never been specific about the threats faced by satellites in geostationary orbit, but they have repeatedly said China and Russia are developing so-called counterspace capabilities.

Mike Gruss covers military space issues, including the U.S. Air Force and Missile Defense Agency, for SpaceNews. He is a graduate of Miami University in Oxford, Ohio.